Optical sheets used for display device surfaces have layers with functions such as an anti-glare property, antistatic property and antifouling property laminated as functional layers on the observer side of a transparent base material. To exhibit these functions, in order to impart an anti-glare property, for example, methods of forming a concavoconvex shape in the surface layer or adding diffusion particles to the resin forming the surface layer are employed. Conductive fine particles or a conductive resin may be added to impart an antistatic property, or a fluorine-containing polymer or stain-proofing agent may be added in order to impart an antifouling property. Since such diffusion particles, conductive fine particles and additives are not completely phase-soluble with surface layer-forming resins, an optical sheet that employs them has a function of diffusing visible light. The concavoconvex sections of the surface layer also have the function of diffusing visible light.
In addition, concavoconvexities larger than the visible light wavelength are formed in the surface layer, the transparent base material back side and between each layer in order to prevent interference patterns between optical sheets and interference patterns between optical sheets and display devices, and such concavoconvexities also have the function of diffusing visible light.
According to the invention, such causes of visible light diffusion are defined as “diffusion factors”, and the presence of such diffusion factors causes the optical sheet to have reduced contrast due to reflection of external light. In other words, an optical sheet should maintain the function of the optical sheet while preventing loss of contrast.
The haze value or the ratio of the interior haze and total haze, is commonly used as a simple method for evaluating contrast. Specifically, it has been considered that an optical sheet with low contrast reduction can be produced by specifying the materials and controlling the production conditions in the optical sheet production process for a lower haze value (see Patent documents 1-3).